<optgroup>

<label for="dino-select">Choose a dinosaur:</label>
<select id="dino-select">
  <optgroup label="Theropods">
    <option>Tyrannosaurus</option>
    <option>Velociraptor</option>
    <option>Deinonychus</option>
  </optgroup>
  <optgroup label="Sauropods">
    <option>Diplodocus</option>
    <option>Saltasaurus</option>
    <option>Apatosaurus</option>
  </optgroup>
</select>

label {
  display: block;
  margin-bottom: 10px;
}

<select>
  <optgroup label="Group 1">
    <option>Option 1.1</option>
  </optgroup>
  <optgroup label="Group 2">
    <option>Option 2.1</option>
    <option>Option 2.2</option>
  </optgroup>
  <optgroup label="Group 3" disabled>
    <option>Option 3.1</option>
    <option>Option 3.2</option>
    <option>Option 3.3</option>
  </optgroup>
</select>